Drive circuit

ABSTRACT

A drive circuit including a signal source that outputs an AC signal, a voltage generator circuit that includes a differential amplifier that generates a first AC voltage with a constant amplitude from the AC signal and outputs the first AC voltage to one end of an external load, a voltage-to-current converter circuit that supplies an AC current with a constant amplitude in opposite phase to the first AC voltage to another end of the external load in accordance with the AC signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional Application of U.S. patentapplication Ser. No. 14/622,618, filed on Feb. 13, 2015, which is basedon Japanese patent application No. 2014-031371, filed on Feb. 21, 2014,the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present invention relates to a drive circuit and, for example, to adrive circuit that drives a load connected thereto.

A drive circuit for driving an external load connected thereto ismounted on various electric equipment, semiconductor devices and thelike.

For example, a drive circuit to be used for bioelectrical impedancemeasurement is proposed (Japanese Unexamined Patent ApplicationPublication No. 2013-12868). A living body is electrically connected asa load to this drive circuit. In this configuration, the drive circuitpasses an alternating current (AC) with a constant amplitude through theliving body and thereby can measure the impedance of the living body. Inthis example, an output terminal of a differential amplifier in thedrive circuit is connected to one end of the living body, and the otherend of the living body is connected to an inverting input terminal ofthe differential amplifier, thereby forming a feedback circuit.

Besides, a drive circuit that passes a constant current through a load(coil) is proposed (Japanese Unexamined Patent Application PublicationNo. 2003-204231). This drive circuit passes a current through the loadby varying an output voltage with respect to a fixed voltage.

SUMMARY

However, the present inventor has found that the above-described drivecircuits have the following problems. The drive circuit disclosed inJapanese Unexamined Patent Application Publication No. 2013-12868 has anegative feedback circuit using a differential amplifier. Thus, theoperation of the differential amplifier becomes unstable depending onthe impedance of the load connected to the differential amplifier. It istherefore difficult to cope with loads having various impedance levels.

The other problems and novel features of the present invention willbecome apparent from the description of the specification and theaccompanying drawings.

According to one embodiment, a drive circuit includes a signal sourcethat outputs an AC signal, a voltage generator circuit that includes adifferential amplifier that generates a first AC voltage with a constantamplitude from the AC signal and outputs the first AC voltage to one endof an external load, and a voltage-to-current converter circuit that isconnected to another end of the external load and supplies an AC currentwith a constant amplitude in opposite phase to the first AC voltage tothe external load.

According to one embodiment, a drive circuit includes a signal sourcethat outputs an AC signal, a voltage generator circuit that outputs afirst AC voltage with a constant amplitude to one end of an externalload, and a voltage-to-current converter circuit that is connected toanother end of the external load and supplies an AC current with aconstant amplitude in opposite phase to the first AC voltage to theexternal load.

According to the above-described embodiment, it is possible to pass acurrent with a constant amplitude to a load without depending on theimpedance of the load.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, advantages and features will be moreapparent from the following description of certain embodiments taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a circuit block diagram showing a configuration of a drivecircuit according to a first embodiment.

FIG. 2 is a circuit diagram showing a configuration of a drive circuitaccording to the first embodiment.

FIG. 3 is a graph showing a relationship between a reference voltage andan output voltage in a drive circuit 100 according to the firstembodiment.

FIG. 4 is a graph showing a differential voltage between an outputvoltage and a reference voltage in a drive circuit according to thefirst embodiment.

FIG. 5 is a graph showing a current flowing through a load by a drivecircuit according to the first embodiment.

FIG. 6 is a graph showing an example of an output voltage of avoltage-to-current converter circuit when a reference voltage isconstant.

FIG. 7 is a graph showing a differential voltage between a constantreference voltage and an output voltage of a voltage-to-currentconverter circuit.

FIG. 8 is a graph showing a current flowing through a load when areference voltage is constant.

FIG. 9 is a circuit diagram showing a configuration of a drive circuitaccording to a second embodiment.

FIG. 10 is a circuit diagram showing a configuration of a drive circuitaccording to a third embodiment.

FIG. 11 is a graph showing a relationship between a reference voltageand an output voltage in a drive circuit according to the thirdembodiment.

FIG. 12 is a circuit diagram showing a configuration of a drive circuitaccording to a fourth embodiment.

FIG. 13 is a circuit diagram showing a configuration of avoltage-to-current converter circuit according to a fifth embodiment.

FIG. 14 is a circuit diagram showing a configuration of avoltage-to-current converter circuit according to a sixth embodiment.

FIG. 15 is a circuit diagram showing a configuration of avoltage-to-current converter circuit according to a seventh embodiment.

FIG. 16 is a circuit diagram showing a configuration of avoltage-to-current converter circuit according to an eighth embodiment.

FIG. 17 is a circuit diagram showing a configuration of avoltage-to-current converter circuit according to a ninth embodiment.

FIG. 18 is a circuit diagram showing a configuration of avoltage-to-current converter circuit according to a tenth embodiment.

FIG. 19 is a circuit diagram showing a configuration of a drive circuitaccording to an eleventh embodiment.

FIG. 20 is a circuit diagram showing a configuration of a drive circuitaccording to the eleventh embodiment.

FIG. 21 is a circuit diagram showing a configuration of a phase adjusteraccording to the eleventh embodiment.

FIG. 22 is a circuit diagram showing a configuration of a phase adjusteraccording to the eleventh embodiment.

FIG. 23 is a circuit diagram showing a configuration of a phase adjusteraccording to the eleventh embodiment.

FIG. 24 is a circuit diagram showing a configuration of a phase adjusteraccording to the eleventh embodiment.

FIG. 25 is a circuit diagram showing a configuration of a drive circuitaccording to the eleventh embodiment.

FIG. 26 is a circuit diagram showing a configuration of a drive circuitaccording to the eleventh embodiment.

FIG. 27 is a circuit diagram showing a configuration of a drive circuitaccording to the eleventh embodiment.

FIG. 28 is a circuit diagram showing a configuration of a drive circuitaccording to the eleventh embodiment.

FIG. 29 is a circuit diagram showing a configuration of a drive circuitaccording to a twelfth embodiment.

FIG. 30 is a circuit diagram showing a configuration of a drive circuitaccording to a thirteenth embodiment.

FIG. 31 is a circuit diagram showing a configuration of a drive circuitaccording to a fourteenth embodiment.

FIG. 32 is a circuit diagram showing a configuration of a drive circuitaccording to a fifteenth embodiment.

DETAILED DESCRIPTION

The preferred embodiments of the present invention will be describedhereinafter in detail with reference to the drawings. It is noted thatin the description of the drawings the same elements will be denoted bythe same reference symbols and redundant description will be omitted.

First Embodiment

A drive circuit 100 according to a first embodiment is describedhereinafter. FIG. 1 is a circuit block diagram showing a configurationof the drive circuit 100 according to the first embodiment. The drivecircuit 100 includes an AC voltage signal source 1, a voltage-to-currentconverter circuit 2, and a reference voltage generator circuit 3. Notethat the AC voltage signal source is also referred to simply as a signalsource. The reference voltage generator circuit is also referred to as avoltage generator circuit.

The AC voltage signal source 1 outputs an AC voltage. One terminal T2 ofthe AC voltage signal source 1 is connected to a positive phase inputterminal of the voltage-to-current converter circuit 2, and the otherterminal T1 is connected to a negative phase input terminal of thevoltage-to-current converter circuit 2. Further, a DC power supply 5that outputs a DC voltage V1 (which is also referred to as a specifiedvoltage) with a level of VDD/2 is inserted between the terminal T1 ofthe AC voltage signal source 1 and the ground. A high-voltage terminalof the DC power supply 5 is connected to the terminal T1 of the ACvoltage signal source 1, and a low-voltage terminal is connected to theground. In this example, a voltage output from the terminal T1 is a DCvoltage V1, and a voltage output from the terminal T2 is an AC signalV2.

The voltage-to-current converter circuit 2 is a circuit that outputs acurrent signal proportional to (i.e. a current signal in phase with) theinput AC signal V2 to a load 4. The positive phase input terminal of thevoltage-to-current converter circuit 2 is connected to the terminal T2of the AC voltage signal source 1. The negative phase input terminal ofthe voltage-to-current converter circuit 2 is connected to the terminalT1 of the AC voltage signal source 1 and the high-voltage terminal ofthe DC power supply 5. The output terminal of the voltage-to-currentconverter circuit 2 is connected to one end of the load 4. Because thevoltage-to-current converter circuit 2 outputs a current signal that isin phase with the AC signal V2, an output voltage V22 (which is alsoreferred to as a first AC voltage) of the voltage-to-current convertercircuit 2 is in phase with the AC voltage V2. Note that thevoltage-to-current converter circuit 2 is inserted between the powersupply voltage VDD and the ground and thereby receives power supply.

The reference voltage generator circuit 3 generates a reference voltageV21 (which is also referred to as a second AC voltage) for feeding acurrent to the load 4. The reference voltage generator circuit 3 isconfigured as an inverting amplifier in this example. The AC signal V2is input to an input terminal of the reference voltage generator circuit3. An output terminal of the reference voltage generator circuit 3 isconnected to the other end of the load 4. The reference voltagegenerator circuit 3 is inserted between the power supply voltage VDD andthe ground and thereby receives power supply.

In this example, the AC signal V2 is input to the positive phase inputterminal of the voltage-to-current converter circuit 2, and the DCvoltage V1 is input to the negative phase input terminal of thevoltage-to-current converter circuit 2. The AC signal V2 is input to theinput terminal of the reference voltage generator circuit 3. As aresult, the reference voltage V21 that is output from the referencevoltage generator circuit 3 is an AC voltage that is in opposite phaseto the output voltage V22 of the voltage-to-current converter circuit 2.

Note that the reference voltage generator circuit may be configured as anon-inverting amplifier, and an AC signal that is in opposite phase tothe AC signal V2 may be input to the input terminal of the referencevoltage generator circuit. Further, the reference voltage generatorcircuit may have another configuration as long as it can output areference voltage in opposite phase to the output voltage of thevoltage-to-current converter circuit.

In the drive circuit 100 having the above-described configuration, whenthe amplitude of the AC signal V2 output from the AC voltage signalsource 1 is positive, the amplitude of the reference voltage V21 isnegative, and the amplitude of the output voltage V22 is positive. Inthis case, a current that flows from the voltage-to-current convertercircuit 2 through the load 4 to the reference voltage generator circuit3 passes through the load 4. Further, in the voltage-to-currentconverter circuit 2, when the amplitude of the AC signal V2 output fromthe AC voltage signal source 1 is negative, the amplitude of thereference voltage V21 is positive, and the amplitude of the outputvoltage V22 is negative. In this case, a current that flows from thereference voltage generator circuit 3 through the load 4 to thevoltage-to-current converter circuit 2 passes through the load 4.Accordingly, a current proportional to the AC signal V2 flows throughthe load 4.

Next, a configuration of the reference voltage generator circuit 3 isdescribed hereinafter. FIG. 2 is a circuit diagram showing aconfiguration of the drive circuit 100 according to the firstembodiment. In this example, the reference voltage generator circuit 3is composed of a resistor R1 (which is also referred to as a firstresistor), a resistor R2 (which is also referred to as a secondresistor) and a differential amplifier AMP. In this example, a phaseadjuster 6 is inserted between the reference voltage generator circuit 3and the AC voltage signal source 1. Note that the phase adjuster 6 isnot an essential element.

The non-inverting input terminal of the differential amplifier AMP isconnected to the terminal T2 of the AC voltage signal source 1 and thepositive phase input terminal of the voltage-to-current convertercircuit 2 through the phase adjuster 6. The inverting input terminal ofthe differential amplifier AMP is connected to the terminal T1 of the ACvoltage signal source 1 and the high-voltage terminal of the DC powersupply 5 through the resistor R1. Further, the inverting input terminalof the differential amplifier AMP is connected to the output terminal ofthe differential amplifier AMP through the resistor R2.

A specific example of the operation of the drive circuit 100 isdescribed hereinafter. The AC voltage signal source 1 receives powersupply with VDD/2 and outputs an AC sinusoidal signal with a frequencyof f=50 kHz and an amplitude of A=1V. If VDD=2.4V, the AC signal V2 isthe AC sinusoidal signal that varies in the range of 1.2V±1.0V.

Hereinafter, the resistances of the resistor R1 and the resistor R2 aredenoted as R1 and R2, respectively. If R1=10 kΩ and R2=9 kΩ, thereference voltage V21 is represented by the following equation (1). Inthe subsequence equations, t indicates time.

$\begin{matrix}\lbrack {{Equation}{\mspace{11mu} \;}1} \rbrack & \; \\{\mspace{79mu} \begin{matrix}{{V\; 21} = {{{- \frac{R\; 2}{R\; 1}} \cdot A \cdot {\sin ( {2\; \pi \; f\; t} )}} + 1.2}} \\{= {{{- 0.9}\; {\sin ( {10^{5}\pi \; t} )}} + 1.2}}\end{matrix}} & (1)\end{matrix}$

If the resistance of the load 4 is RT=2.5 kΩ and the amplitude of theoutput current I of the voltage-to-current converter circuit 2 is 800mA, the output voltage V22 of the voltage-to-current converter circuit 2is represented by the following equation (2).

$\begin{matrix}\lbrack {{Equation}\mspace{14mu} 2} \rbrack & \; \\{\mspace{79mu} \begin{matrix}{{V\; 22} = {{{RT} \cdot I \cdot {\sin ( {2\; \pi \; f\; t} )}} + {V\; 21}}} \\{= {{2\; {\sin ( {10^{5}\pi \; t} )}} - {0.9\; {\sin ( {10^{5}\pi \; t} )}} + 1.2}} \\{= {{1.1\; {\sin ( {10^{5}\pi \; t} )}} + 1.2}}\end{matrix}} & (2)\end{matrix}$

FIG. 3 is a graph showing a relationship between the reference voltageV21 and the output voltage V22 in the drive circuit 100 according to thefirst embodiment. As shown in FIG. 3, the reference voltage V21 variesin the range of +0.3V to +2.1V and the output voltage V22 varies in therange of +0.1V to +2.3V.

FIG. 4 is a graph showing a differential voltage between the outputvoltage V22 and the reference voltage V21 in the drive circuit 100according to the first embodiment. Because the output voltage V22 andthe reference voltage V21 are in opposite phase to each other, adifferential voltage ΔV between the output voltage V22 and the referencevoltage V21, which serves as a drive voltage for passing a currentthrough the load 4, can vary in the range of −2.0V to +2.0V.Accordingly, in the drive circuit 100, the both amplitude levels(4.0Vp-p in this example) of the voltage applied to the load can beincreased to be higher than the power supply voltage VDD (+2.4V).

FIG. 5 is a graph showing a current flowing through a load by the drivecircuit 100 according to the first embodiment. Because the drive circuit100 can increase the amplitude of the voltage applied to the load, thecurrent flowing through the load can be high (amplitude of 800 μA) evenat a low power supply voltage.

As a comparative example, the case where the reference voltage isconstant (eg. Japanese Unexamined Patent Application Publication No.2003-204231) is described. It is assumed that the reference voltage is+1.2V, which is ½ of the power supply voltage (+2.4V), for example. FIG.6 is a graph showing an example of the output voltage of thevoltage-to-current converter circuit when the reference voltage isconstant. In this case, if the amplitude of the voltage applied to theload is 2.0V, which is the same as in the drive circuit 100, the outputvoltage Vout of the voltage-to-current converter circuit needs to varyin the range of −0.8V to +3.2V with respect to the reference voltageVref (the dashed line in FIG. 6). However, the output voltage Vout ofthe voltage-to-current converter circuit cannot be higher than the powersupply voltage (+2.4V) and lower than the ground (0V). Therefore, theoutput voltage Vout is a waveform where the top of a sinusoidal wave iscut as shown by the solid line in FIG. 6.

FIG. 7 is a graph showing a differential voltage Vd between a constantreference voltage and an output voltage of a voltage-to-currentconverter circuit. FIG. 8 is a graph showing a current flowing through aload when a reference voltage is constant. As described above, becausethe output voltage Vout is a waveform where the top of a sinusoidal waveis cut, the differential voltage Vd is also a waveform where the top ofa sinusoidal wave is cut. Accordingly, a current flowing through theload (2.5 kΩ) is also a waveform where the top of a sinusoidal wave iscut, and the current amplitude is limited to 480 μA.

Therefore, according to this configuration, since the reference voltageis an opposite phase to the output voltage of the voltage-to-currentconverter circuit, it is possible to pass a larger current compared withthe case where the reference voltage is constant.

Note that, as a technique of using two amplifiers in parallel with eachother, BTL (Bridged Transless) connection is generally known. However,while the BTL connection is a configuration that merely increases theoutput, this configuration increases the current output to the load andfurther decreases the output voltage amplitude of the voltage-to-currentconverter circuit compared with the case where the reference voltage isconstant. It is thus understood that this configuration is different inoperation from the BTL connection.

Second Embodiment

A drive circuit 200 according to a second embodiment is describedhereinafter. FIG. 9 is a circuit diagram showing a configuration of thedrive circuit 200 according to the second embodiment. The drive circuit200 has a configuration where the reference voltage generator circuit 3in the drive circuit 100 is replaced with a reference voltage generatorcircuit 31. The other configuration of the drive circuit 200 is the sameas that of the drive circuit 100 and thus not redundantly described.

The reference voltage generator circuit 31 is composed of a resistor R1(which is also referred to as a first resistor), a resistor R2 (which isalso referred to as a second resistor) and a differential amplifier AMP.

The non-inverting input terminal of the differential amplifier AMP isconnected to the terminal T1 of the AC voltage signal source 1 and thehigh-voltage terminal of the DC power supply 5. The inverting inputterminal of the differential amplifier AMP is connected to the terminalT2 of the AC voltage signal source 1 and the positive phase inputterminal of the voltage-to-current converter circuit 2 through theresistor R1. Further, the inverting input terminal of the differentialamplifier AMP is connected to the output terminal of the differentialamplifier AMP through the resistor R2.

A specific example of the operation of the drive circuit 200 isdescribed hereinafter. As in the first embodiment, the reference voltageV21 is represented by the equation (1) and the output voltage V22 isrepresented by the equation (2). In the drive circuit 200, just like inthe drive circuit 100 (FIG. 3), the reference voltage V21 and the outputvoltage V22 are in opposite phase and vary in the same manner.

It is thus understood that the drive circuit 200 can pass a currentthrough a load just like the drive circuit 100, although theconfiguration of the reference voltage generator circuit is different.

Third Embodiment

A drive circuit 300 according to a third embodiment is describedhereinafter. FIG. 10 is a circuit diagram showing a configuration of thedrive circuit 300 according to the third embodiment. The drive circuit300 has a configuration where the reference voltage generator circuit 3in the drive circuit 100 is replaced with a reference voltage generatorcircuit 32. The other configuration of the drive circuit 300 is the sameas that of the drive circuit 100 and not redundantly described.

The reference voltage generator circuit 32 is composed of a differentialamplifier AMP.

The non-inverting input terminal of the differential amplifier AMP isconnected to the terminal T1 of the AC voltage signal source 1 and thehigh-voltage terminal of the DC power supply 5. The inverting inputterminal of the differential amplifier AMP is connected to the terminalT2 of the AC voltage signal source 1 and the positive phase inputterminal of the voltage-to-current converter circuit 2.

A specific example of the operation of the drive circuit 300 isdescribed hereinafter. FIG. 11 is a graph showing a relationship betweenthe reference voltage V21 and the output voltage V22 in the drivecircuit 300 according to the third embodiment. As shown in FIG. 11, thereference voltage V21 output from the reference voltage generatorcircuit 32 is a rectangular wave with a voltage varying at 0V or +2.4Vaccording to a change in the relationship in level between the AC signalV2 and the DC voltage V1.

On the other hand, the output voltage V22 is a sinusoidal wave. When thereference voltage V21 is 0V, the output voltage V22 is a waveform thatis projecting upward with an amplitude of 2.0V with reference to thereference voltage V21 (0V). On the other hand, when the referencevoltage V21 is +2.4V, the output voltage V22 is a waveform that isprojecting downward with an amplitude of 2.0V with reference to thereference voltage V21 (+2.4V). Thus, a differential voltage ΔV betweenthe reference voltage V21 and the output voltage V22 varies in the samemanner as in the drive circuit 100 (FIG. 4).

As described above, according to this configuration, a voltage appliedto the load 4 is the same as in the drive circuit 100, although thewaveforms of the reference voltage V21 and the output voltage V22 aredifferent. Thus, the drive circuit 300 can supply a current to the load4 just like the drive circuit 100. Further, in the drive circuit 300,the configuration of the reference voltage generator circuit can besimplified compared with that of the drive circuit 100.

Fourth Embodiment

A drive circuit 400 according to a fourth embodiment is describedhereinafter. FIG. 12 is a circuit diagram showing a configuration of thedrive circuit 400 according to the fourth embodiment. The drive circuit400 has a configuration where the reference voltage generator circuit 3in the drive circuit 100 is replaced with a reference voltage generatorcircuit 33. The other configuration of the drive circuit 400 is the sameas that of the drive circuit 100 and thus not redundantly described.

The reference voltage generator circuit 33 is composed of an inverterINV. An input terminal of the inverter INV is connected to the terminalT2 of the AC voltage signal source 1. An output terminal of the inverterINV is connected to the load 4.

A specific example of the operation of the drive circuit 400 isdescribed hereinafter. The reference voltage V21 output from theinverter INV is 0V when the AC signal V2 is positive and it is +2.4Vwhen the AC signal V2 is negative. Thus, the reference voltage V21 inthe drive circuit 400 is the same waveform as in the drive circuit 300(FIG. 11). As a result, the output voltage V22 in the drive circuit 400is also the same waveform as in the drive circuit 300 (FIG. 11).

As described above, according to this configuration, a voltage appliedto the load 4 is the same as in the drive circuit 300, although theconfiguration of the reference voltage generator circuit 3 is different.Thus, the drive circuit 400 can supply a current to the load 4 just likethe drive circuit 100.

Fifth Embodiment

A voltage-to-current converter circuit according to a fifth embodimentis described hereinafter. A voltage-to-current converter circuit 21described in this embodiment is a specific example of thevoltage-to-current converter circuit 2 described above. FIG. 13 is acircuit diagram showing a configuration of the voltage-to-currentconverter circuit 21 according to the fifth embodiment.

The voltage-to-current converter circuit 21 includes Pch transistors MP1to MP7, Nch transistors MN1 to MN4, a resistor R11, and a current sourceIREF.

The power supply voltage VDD is supplied to the sources of the Pchtransistors MP1 to MP5. The current source IREF is inserted between thedrain and the ground of the Pch transistor MP1. The drain of the Pchtransistor MP2 is connected to the source of the Pch transistor MP6. Thedrain of the Pch transistor MP3 is connected wo the source of the Pchtransistor MP7. The gates of the Pch transistors MP1 to MP3 and thedrain of the Pch transistor MP1 are respectively connected to eachother.

The drain of the Pch transistor MP6 is connected to the drain of the Nchtransistor MN1. The drain of the Pch transistor MP7 is connected to thedrain of the Nch transistor MN2. The resistor R11 is connected betweenthe drain of the Pch transistor MP6 and the drain of the Pch transistorMP7. The DC voltage V1 is applied to the gate of the Pch transistor MP6.The AC signal V2 is applied to the gate of the Pch transistor MP7. Thesource of the Nch transistor MN1 is connected to the ground. The sourceof the Nch transistor MN2 is connected to the ground.

The drain of the Pch transistor MP4 is connected to the drain of the Nchtransistor MN3. The source of the Nch transistor MN3 is connected to theground. The drain of the Pch transistor MP5 is connected to the drain ofthe Nch transistor MN4. The source of the Nch transistor MN4 isconnected to the ground. The drain of the Pch transistor MP4 and thegates of the Pch transistor MP4 and MP5 are connected to each other.

The gate of the Nch transistor MN1, the drain of the Nch transistor MN1and the gate of the Nch transistor MN4 are connected to each other. Thegate of the Nch transistor MN2, the drain of the Nch transistor MN2 andthe gate of the Nch transistor MN3 are connected to each other.

A node between the drain of the Pch transistor MP5 and the drain of theNch transistor MN4 is connected to the output terminal TOUT. The outputvoltage V22 is output from the output terminal TOUT.

The Nch transistor MN1 and the Nch transistor MN2 are the transistors ofthe same size. The Nch transistor MN3 and the Nch transistor MN4 are thetransistors of the same size. Further, the size ratio (S2/S1) betweenthe size S1 of the Nch transistors MN1 and MN2 and the size S2 of theNch transistors MN3 and MN4 is denoted by M (M is a positive realnumber).

The output current I of the voltage-to-current converter circuit 21 isrepresented by the following equation (3).

$\begin{matrix}\lbrack {{Equation}\mspace{14mu} 3} \rbrack & \; \\{\mspace{79mu} {I = {M\frac{{V\; 2} - {V\; 1}}{R\; 11}}}} & (3)\end{matrix}$

As described above, the voltage-to-current converter circuit thatoutputs a current in accordance with the resistance of the resistor R11,the DC voltage V1, the AC signal V2 and the transistor size ratio can bespecifically configured.

Sixth Embodiment

A voltage-to-current converter circuit according to a sixth embodimentis described hereinafter. A voltage-to-current converter circuit 22described in this embodiment is a specific example of thevoltage-to-current converter circuit 2 described above. FIG. 14 is acircuit diagram showing a configuration of the voltage-to-currentconverter circuit 22 according to the sixth embodiment.

The voltage-to-current converter circuit 22 includes resistors 221 to225 and a differential amplifier 226. The AC signal V2 is applied to oneend of the resistor 221, and the other end is connected to the invertinginput terminal of the differential amplifier 226. The resistor 222 isconnected between the inverting input terminal of the differentialamplifier 226 and the output terminal of the differential amplifier 226.The resistor 223 is connected between the output terminal of thedifferential amplifier 226 and the output terminal TOUT of thevoltage-to-current converter circuit 22. The DC voltage V1 is applied toone end of the resistor 224, and the other end is connected to thenon-inverting input terminal of the differential amplifier 226 and oneend of the resistor 225. The other end of the resistor 225 is connectedto the output terminal TOUT of the voltage-to-current converter circuit22. The output voltage V22 is output from the output terminal TOUT.

The resistances of the resistor 221 and the resistor 224 are denoted byRs. The resistances of the resistor 222 and the resistor 225 are denotedby Rf. The resistance of the resistor 223 is denoted by R0. Then, theoutput current I of the voltage-to-current converter circuit 22 isrepresented by the following equation (4).

$\begin{matrix}\lbrack {{Equation}\mspace{14mu} 4} \rbrack & \; \\{\mspace{79mu} {I = {{- \frac{R\; f}{R\; s}} \cdot \frac{{V\; 2} - {V\; 1}}{R\; 0}}}} & (4)\end{matrix}$

As described above, the voltage-to-current converter circuit thatoutputs a current in accordance with the resistances of the resistors221 to 225, the DC voltage V1 and the AC signal V2 can be specificallyconfigured.

Seventh Embodiment

A voltage-to-current converter circuit according to a seventh embodimentis described hereinafter. A voltage-to-current converter circuit 23described in this embodiment is a specific example of thevoltage-to-current converter circuit 2 described above. FIG. 15 is acircuit diagram showing a configuration of the voltage-to-currentconverter circuit 23 according to the seventh embodiment.

The voltage-to-current converter circuit 23 is a modified example of thevoltage-to-current converter circuit 22. While the AC signal V2 isapplied to the resistor 221 in the voltage-to-current converter circuit22, the DC voltage V1 is applied to the resistor 221 in thevoltage-to-current converter circuit 23. While the DC voltage V1 isapplied to the resistor 224 in the voltage-to-current converter circuit22, the AC signal V2 is applied to the resistor 224 in thevoltage-to-current converter circuit 23. The other configuration of thevoltage-to-current converter circuit 23 is the same as that of thevoltage-to-current converter circuit 22 and thus not redundantlydescribed.

The output current I of the voltage-to-current converter circuit 23 isrepresented by the following equation (5).

$\begin{matrix}\lbrack {{Equation}\mspace{14mu} 5} \rbrack & \; \\{\mspace{79mu} {I = {{- \frac{R\; f}{R\; s}} \cdot \frac{{V\; 2} - {V\; 1}}{R\; 0}}}} & (5)\end{matrix}$

As described above, the voltage-to-current converter circuit thatoutputs a current in accordance with the resistances of the resistors221 to 225, the DC voltage V1 and the AC signal V2 can be specificallyconfigured.

Eighth Embodiment

A voltage-to-current converter circuit according to an eighth embodimentis described hereinafter. A voltage-to-current converter circuit 24described in this embodiment is a specific example of thevoltage-to-current converter circuit 2 described above. FIG. 16 is acircuit diagram showing a configuration of the voltage-to-currentconverter circuit 24 according to the eighth embodiment.

The voltage-to-current converter circuit 24 has a configuration in whicha differential amplifier 241 is added to the voltage-to-currentconverter circuit 22. The DC voltage V1 is applied to one end of theresistor 224, and the other end is connected to the non-inverting inputterminal of the differential amplifier 226 and one end of the resistor225. The other end of the resistor 225 is connected to the outputterminal and the inverting input terminal of the differential amplifier241. The non-inverting input terminal of the differential amplifier 241is connected to the output terminal TOUT of the voltage-to-currentconverter circuit 22. The output voltage V22 is output from the outputterminal TOUT. The other configuration of the voltage-to-currentconverter circuit 24 is the same as that of the voltage-to-currentconverter circuit 22 and thus not redundantly described.

The output current I of the voltage-to-current converter circuit 24 isrepresented by the above-described equation (4). As described above, thevoltage-to-current converter circuit that outputs a current inaccordance with the resistances of the resistors 221 to 225, the DCvoltage V1 and the AC signal V2 can be specifically configured.

Ninth Embodiment

A voltage-to-current converter circuit according to a ninth embodimentis described hereinafter. A voltage-to-current converter circuit 25described in this embodiment is a specific example of thevoltage-to-current converter circuit 2 described above. FIG. 17 is acircuit diagram showing a configuration of the voltage-to-currentconverter circuit 25 according to the ninth embodiment.

The voltage-to-current converter circuit 25 is a modified example of thevoltage-to-current converter circuit 24. While the AC signal V2 isapplied to the resistor 221 in the voltage-to-current converter circuit24, the DC voltage V1 is applied to the resistor 221 in thevoltage-to-current converter circuit 25. While the DC voltage V1 isapplied to the resistor 224 in the voltage-to-current converter circuit24, the AC signal V2 is applied to the resistor 224 in thevoltage-to-current converter circuit 25. The other configuration of thevoltage-to-current converter circuit 25 is the same as that of thevoltage-to-current converter circuit 24 and thus not redundantlydescribed.

The output current I of the voltage-to-current converter circuit 25 isrepresented by the above-described equation (5). As described above, thevoltage-to-current converter circuit that outputs a current inaccordance with the resistances of the resistors 221 to 225, the DCvoltage V1 and the AC signal V2 can be specifically configured.

Tenth Embodiment

A voltage-to-current converter circuit according to a tenth embodimentis described hereinafter. A voltage-to-current converter circuit 26described in this embodiment is a specific example of thevoltage-to-current converter circuit 2 described above. FIG. 18 is acircuit diagram showing a configuration of the voltage-to-currentconverter circuit 26 according to the tenth embodiment.

The voltage-to-current converter circuit 26 includes a Pch transistorMP10, an Nch transistor MN10, resistors R21 and R22, and differentialamplifiers 261 and 262.

The power supply voltage VDD is applied to one end of the resistor R21,and the other end is connected to the source of the Pch transistor MP10.The drain of the Pch transistor MP10 is connected to the drain of theNch transistor MN10. One end of the resistor R22 is connected to theground, and the other and is connected to the source of the Nchtransistor MN10. A node between the drain of the Pch transistor MP10 andthe drain of the Nch transistor MN10 is connected to the output terminalTOUT. The output voltage V22 is output from the output terminal TOUT.

The AC signal V2 is applied to the non-inverting input terminal of thedifferential amplifier 261. The inverting input terminal of thedifferential amplifier 261 is connected to the source of the Pchtransistor MP10. The output terminal of the differential amplifier 261is connected to the gate of the Pch transistor MP10. The non-invertinginput terminal of the differential amplifier 261 corresponds to thepositive phase input terminal of the voltage-to-current convertercircuit 26.

The DC voltage V1 is applied to the non-inverting input terminal of thedifferential amplifier 262. The inverting input terminal of thedifferential amplifier 262 is connected to the source of the Nchtransistor MN10. The output terminal of the differential amplifier 262is connected to the gate of the Nch transistor MN10. The non-invertinginput terminal of the differential amplifier 262 corresponds to thenegative phase input terminal of the voltage-to-current convertercircuit 26.

Eleventh Embodiment

A phase adjuster according to an eleventh embodiment is describedhereinafter. A phase adjuster described in this embodiment is a modifiedexample of the phase adjuster 6 described above.

A phase adjuster 61, which is a first modified example of the phaseadjuster, is described hereinafter. FIG. 19 is a circuit diagram showinga configuration of a drive circuit 501 according to the eleventhembodiment. The drive circuit 501 has a configuration in which the phaseadjuster 61 is added to the drive circuit 200 shown in FIG. 9.

The phase adjuster 61 includes a resistor R61 and a capacitor C1 and isconfigured as a passive low-pass filter. The resistor R61 is insertedbetween the terminal T2 of the AC voltage signal source 1 and thepositive phase input terminal of the voltage-to-current convertercircuit 2, and the resistor R1 in the reference voltage generatorcircuit 31. One end of the capacitor C1 is connected to a node betweenthe resistor R61 and the resistor R1 in the reference voltage generatorcircuit 31. The other end of the capacitor C1 is connected to thenon-inverting input terminal of the differential amplifier AMP in thereference voltage generator circuit 31.

Because the phase adjuster 61 is a low-pass filter, it is possible tomake an adjustment to delay the phase of the reference voltage V21 inthis configuration.

The phase adjuster 62, which is a second modified example of the phaseadjuster, is described hereinafter. FIG. 20 is a circuit diagram showinga configuration of a drive circuit 502 according to the eleventhembodiment. The phase adjuster 62 includes a resistor R62 and acapacitor C2 and is configured as a passive high-pass filter. Thecapacitor C2 is inserted between the terminal T2 of the AC voltagesignal source 1 and the positive phase input terminal of thevoltage-to-current converter circuit 2, and the resistor R1 in thereference voltage generator circuit 31. One end of the resistor R62 isconnected to a node between the capacitor C2 and the resistor R1 in thereference voltage generator circuit 31. The other end of the resistorR62 is connected to the non-inverting input terminal of the differentialamplifier AMP in the reference voltage generator circuit 31.

Because the phase adjuster 62 is a high-pass filter, it is possible tomake an adjustment to advance the phase of the reference voltage V21 inthis configuration.

A phase adjuster 63, which is a third modified example of the phaseadjuster, is described hereinafter. FIG. 21 is a circuit diagram showinga configuration of the phase adjuster 63, which one example of the phaseadjuster according to the eleventh embodiment. The phase adjuster 63includes a differential amplifier 630, resistors 631 and 632, andcapacitors 633 and 634, and is configured as an active low-pass filter.

The AC signal V2 is applied to one end of the resistor 631. The otherend of the resistor 631 is connected to one end of the resistor 632 andone end of the capacitor 633. The other end of the resistor 632 isconnected to the non-inverting input terminal of the differentialamplifier 630 and one end of the capacitor 634. The other end of thecapacitor 633 is connected to the output terminal of the differentialamplifier 630. The DC voltage V1 is applied to the other end of thecapacitor 634. The inverting input terminal of the differentialamplifier 630 is connected to the output terminal of the differentialamplifier 630. Further, the output terminal of the differentialamplifier 630 is connected to the terminal T3. The terminal T3 isconnected to the reference voltage generator circuit.

Because the phase adjuster 63 is a low-pass filter, it is possible tomake an adjustment to delay the phase of the reference voltage V21 inthis configuration.

A phase adjuster 64, which is a fourth modified example of the phaseadjuster, is described hereinafter. FIG. 22 is a circuit diagram showinga configuration of the phase adjuster 64, which one example of the phaseadjuster according to the eleventh embodiment. The phase adjuster 64 hasa configuration in which resistors 641 and 642 are added to the phaseadjuster 63. One end of the resistor 641 is connected to the invertinginput terminal of the differential amplifier 630, and the DC voltage V1is applied to the other end of the resistor 641. The resistor 642 isinserted between the inverting input terminal of the differentialamplifier 630 and the output terminal of the differential amplifier 630.The other configuration of the phase adjuster 64 is the same as that ofthe phase adjuster 63 and thus not redundantly described.

Because the phase adjuster 64 is a low-pass filter, it is possible tomake an adjustment to delay the phase of the reference voltage V21 inthis configuration.

A phase adjuster 65, which is a fifth modified example of the phaseadjuster, is described hereinafter. FIG. 23 is a circuit diagram showinga configuration of the phase adjuster 65, which one example of the phaseadjuster according to the eleventh embodiment. The phase adjuster 65 isconfigured as an active high-pass filter. The phase adjuster 65 has aconfiguration in which the resistor 631 and the capacitor 633 arereplaced with each other and further the resistor 632 and the capacitor634 are replaced with each other in the phase adjuster 63 describedabove.

Because the phase adjuster 65 is a high-pass filter, it is possible tomake an adjustment to advance the phase of the reference voltage V21 inthis configuration.

A phase adjuster 66, which is a sixth modified example of the phaseadjuster, is described hereinafter. FIG. 24 is a circuit diagram showinga configuration of the phase adjuster 66, which one example of the phaseadjuster according to the eleventh embodiment. The phase adjuster 66 hasa configuration in which resistors 641 and 642 are added to the phaseadjuster 65. The resistors 641 and 642 are the same as those in thephase adjuster 64 and thus not redundantly described.

Because the phase adjuster 66 is a high-pass filter, it is possible tomake an adjustment to advance the phase of the reference voltage V21 inthis configuration.

A phase adjuster 67, which is a seventh modified example of the phaseadjuster, is described hereinafter. FIG. 25 is a circuit diagram showinga configuration of a drive circuit 507 according to the eleventhembodiment. The drive circuit 507 has a configuration in which the phaseadjuster 6 in the drive circuit 100 is replaced by a phase adjuster 67.The phase adjuster 67 includes a differential amplifier 670, resistors671 to 673 and a capacitor 674 and is configured as an all-pass filter.

The AC signal V2 is applied to one end of the resistor 671, and theother end is connected to the inverting input terminal of thedifferential amplifier 670. The AC signal V2 is applied to one end ofthe resistor 672, and the other end is connected to the non-invertinginput terminal of the differential amplifier 670. The resistor 673 isinserted between the inverting input terminal of the differentialamplifier 670 and the output terminal of the differential amplifier 670.Further, the output terminal of the differential amplifier 670 isconnected to the non-inverting input terminal of the differentialamplifier AMP in the reference voltage generator circuit 3. The DCvoltage V1 is applied to one end of the capacitor 674, and the other endis connected to the non-inverting input terminal of the differentialamplifier 670. Note that the differential amplifier 670 is insertedbetween the power supply voltage VDD and the ground and thereby receivespower supply.

As described above, in this configuration, it is possible to make anadjustment to delay the phase of the reference voltage V21. Further,because the above-described phase adjusters 61 to 66 are RC filters, anadjustment of the phase of the reference voltage V21 causes a change inthe voltage amplitude. On the other hand, because the phase adjuster 67is an all-pass filter, it is possible to make an adjustment of the phaseof the reference voltage V21 without change in the voltage amplitude.

A phase adjuster 68, which is an eighth modified example of the phaseadjuster, is described hereinafter. FIG. 26 is a circuit diagram showinga configuration of a drive circuit 508 according to the eleventhembodiment. The drive circuit 508 has a configuration in which the phaseadjuster 6 in the drive circuit 100 is replaced by a phase adjuster 68.The phase adjuster 68 has a configuration in which the resistor 672 andthe capacitor 674 in the phase adjuster 67 are replaced with each other.The other configuration of the phase adjuster 68 is the same as that ofthe phase adjuster 67 and thus not redundantly described.

As described above, in this configuration, it is possible to make anadjustment to advance the phase of the reference voltage V21. Further,because the above-described phase adjusters 61 to 66 are RC filters, anadjustment of the phase of the reference voltage V21 causes a change inthe voltage amplitude. On the other hand, because the phase adjuster 68is an all-pass filter, it is possible to make an adjustment of the phaseof the reference voltage V21 without change in the voltage amplitude.

A phase adjuster 69, which is a ninth modified example of the phaseadjuster, is described hereinafter. FIG. 27 is a circuit diagram showinga configuration of a drive circuit 509 according to the eleventhembodiment. The drive circuit 509 has a configuration in which the phaseadjuster 69 is added to the drive circuit 200. The phase adjuster 69includes n (n is an integer of 1 or more) number of buffers B_1 to B_nthat are in cascade connection.

Each of the buffers B_1 to B_n is composed of a differential amplifier691. The inverting input terminal of the differential amplifier 691 isconnected to the output terminal of the differential amplifier 691.

The AC signal V2 is applied to the non-inverting input terminal of thedifferential amplifier 691 that constitutes the buffer B_1 in the firststage. Likewise, the non-inverting input terminal of the differentialamplifier 691 that constitutes the k-th (k is an integer of 2≦k≦n−1)buffer B_k is connected to the output terminal of the differentialamplifier 691 that constitutes the (k−1)th buffer B_k−1. The outputterminal of the differential amplifier 691 that constitutes the n-thbuffer B_n is connected to the inverting input terminal of the referencevoltage generator circuit 3. Note that the differential amplifier 691that constitute the buffers B_1 to B_n is inserted between the powersupply voltage VDD and the ground and thereby receives power supply.

The phase adjuster 69 having the multiple stage buffers can delay an ACsignal passing through it. As described above, in this configuration, itis possible to make an adjustment to delay the phase of the referencevoltage V21.

A phase adjuster 70, which is a tenth modified example of the phaseadjuster, is described hereinafter. FIG. 28 is a circuit diagram showinga configuration of a drive circuit 510 according to the eleventhembodiment. The drive circuit 510 has a configuration in which the phaseadjuster 70 is added to the drive circuit 200. The phase adjuster 70 hasa configuration in which a connected position of the phase adjuster 69is changed.

The output terminal of the differential amplifier 691 that constitutesthe n-th buffer B_n is connected to the positive phase input terminal ofthe voltage-to-current converter circuit 2. The other configuration ofthe phase adjuster 70 is the same as that of the phase adjuster 69 andthus not redundantly described.

The phase adjuster 70 having the multiple stage buffers can delay an ACsignal passing through it. As described above, in this configuration, itis possible to make an adjustment to advance the phase of the referencevoltage V21.

Twelfth Embodiment

A drive circuit 600 according to a twelfth embodiment is describedhereinafter. FIG. 29 is a circuit diagram showing a configuration of thedrive circuit 600 according to the twelfth embodiment. The drive circuit600 has a configuration in which the AC voltage signal source isprovided with a phase adjustment function, instead of a phase adjuster.Specifically, the drive circuit 600 has a configuration in which thephase adjuster 6 is eliminated from the drive circuit 100 and the ACvoltage signal source 1 is replaced by an AC voltage signal source 10.

The AC voltage signal source 10 outputs an AC signal V12 (which is alsoreferred to as a first AC signal) to the voltage-to-current convertercircuit 2 and outputs an AC signal V11 (which is also referred to as asecond AC signal) to the reference voltage generator circuit 3. The ACvoltage signal source 10 includes a control circuit 11,digital-to-analog converters (DAC) 12 and 13, and low-pass filters (LPF)14 and 15.

The control circuit 11 outputs a digital signal to the DAC 12 and 13 forcontrolling their operation. The DAC 12 and 13 convert the input digitalsignal into an analog signal and thereby output an AC voltage. The ACvoltage output from the DAC 12 enters the LPF 14 where its highfrequency component is eliminated and is then output as the AC signalV12. The AC voltage output from the DAC 13 enters the LPF 15 where itshigh frequency component is eliminated and is then output as the ACsignal V11.

For example, if the control circuit 11 supplies the digital signal tothe DAC 13 later than the digital signal supplied to the DAC 12, it ispossible to delay the phase of the AC signal V11 compared with the ACsignal V12. Further, if the control circuit 11 supplies the digitalsignal to the DAC 13 earlier than the digital signal supplied to the DAC12, it is possible to advance the phase of the AC signal V11 comparedwith the AC signal V12.

Note that the AC voltage signal source 10 can be applied to the drivecircuit according to the embodiments other than the drive circuit 100.

Thirteenth Embodiment

A drive circuit 700 according to a thirteenth embodiment is describedhereinafter. FIG. 30 is a circuit diagram showing a configuration of thedrive circuit 700 according to the thirteenth embodiment. The drivecircuit 700 has a configuration in which the resistors R1 and R2 in thereference voltage generator circuit 3 of the drive circuit 100 arereplaced by variable resistors VR1 and VR2 and further a control circuit7 is added. The control circuit 7 is composed of a digital circuit, forexample, and can control the resistances of the variable resistors VR1and VR2. The other configuration of the drive circuit 700 is the same asthat of the drive circuit 100 and thus not redundantly described.

Hereinafter, the resistances of the variable resistors VR1 and VR2 aredenoted by R3 and R4, respectively. In this case, the reference voltageV21 is represented by the following equation (6), which is amodification of the equation (1).

$\begin{matrix}\lbrack {{Equation}\mspace{14mu} 6} \rbrack & \; \\{\mspace{79mu} {{V\; 21} = {{{- \frac{R\; 4}{R\; 3}} \cdot A \cdot {\sin ( {2\; \pi \; f\; t} )}} + 1.2}}} & (6)\end{matrix}$

In the drive circuit 700, it is possible to control the amplitude of thereference voltage V21 by controlling the resistances of the variableresistors VR1 and VR2 as represented by the equation (6). By controllingthe amplitude of the reference voltage V21, the amplitude of the outputvoltage V22 can be controlled in the same manner.

As described above, according to this configuration, by appropriatelycontrolling the amplitude of the reference voltage V21 and the outputvoltage V22 as initial setting after connecting the load 4, it ispossible to supply a current with a desired amplitude to the load 4.

Fourteenth Embodiment

A drive circuit 800 according to a fourteenth embodiment is describedhereinafter. FIG. 31 is a circuit diagram showing a configuration of thedrive circuit 800 according to the fourteenth embodiment. The drivecircuit 800 has a configuration in which a control circuit 8 is added tothe drive circuit 100. The control circuit 8 controls the phaseadjustment amount of the phase adjuster 6 in the reference voltagegenerator circuit 3. The control circuit 8 monitors the output voltageV22, for example, and outputs a control signal indicating the phaseadjustment amount to the phase adjuster 6 according to a monitoringresult.

As described above, according to this configuration, by appropriatelycontrolling the phase adjustment amount of the phase adjuster 6 asinitial setting after connecting the load 4, it is possible to controlthe reference voltage V21 to be in opposite phase to the output voltageV22.

Fifteenth Embodiment

A drive circuit 900 according to a fifteenth embodiment is describedhereinafter. FIG. 32 is a circuit diagram showing a configuration of thedrive circuit 900 according to the fifteenth embodiment. The drivecircuit 900 is a modified example of the drive circuit 700 and has aconfiguration in which the control circuit 7 is replaced by a controlcircuit 9. The control circuit 9 performs not only control of theresistances of the variable resistors VR1 and VR2 but also control ofthe phase adjustment amount of the phase adjuster 6 just like thecontrol circuit 8.

As described above, according to this configuration, by appropriatelycontrolling the amplitude of the reference voltage V21 and the outputvoltage V22 as initial setting after connecting the load 4, it ispossible to supply a current with a desired amplitude to the load 4. Inaddition, by appropriately controlling the phase adjustment amount ofthe phase adjuster 6 as initial setting after connecting the load 4, itis possible to control the reference voltage V21 to be in opposite phaseto the output voltage V22.

The present invention is not limited to the above-described embodiments,and various changes and modifications may be made without departing fromthe scope of the invention. For example, the above-described phaseadjusters 61, 62, 69 and 70 can be applied to the drive circuitaccording to the above-described embodiments other than the drivecircuit 200. The above-described phase adjusters 67 and 68 can beapplied to the drive circuit according to the above-describedembodiments other than the drive circuit 100.

Although the drive circuits according to the above-described embodiments12 and 14 are described as modified examples of the drive circuit 100,they may be configured as modified examples of the drive circuitaccording to the above-described embodiments other than the drivecircuit 100. Although the drive circuits according to theabove-described embodiments 13 and 15 are described as modified examplesof the drive circuit 100, they may be configured as modified examples ofthe drive circuit 200 according to the embodiment.

As the load 4, various types of loads that require an AC current such asa living body for bioelectrical impedance measurement or a display panelmay be used.

Although embodiments of the present invention are described specificallyin the foregoing, the present invention is not restricted to theabove-described embodiments, and various changes and modifications maybe made without departing from the scope of the invention.

The above-described embodiments can be combined as desirable by one ofordinary skill in the art.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention can bepracticed with various modifications within the spirit and scope of theappended claims and the invention is not limited to the examplesdescribed above.

Further, the scope of the claims is not limited by the embodimentsdescribed above.

Furthermore, it is noted that, Applicant's intent is to encompassequivalents of all claim elements, even if amended later duringprosecution.

What is claimed is:
 1. A drive circuit comprising: a signal source thatoutputs an AC signal; a voltage generator circuit that includes adifferential amplifier that generates a first AC voltage with a constantamplitude from the AC signal and outputs the first AC voltage to one endof an external load; a voltage-to-current converter circuit thatsupplies an AC current with a constant amplitude in opposite phase tothe first AC voltage to another end of the external load in accordancewith the AC signal, wherein the AC signal is a positive voltage varyingwith a constant amplitude with reference to a specified voltage, whereinthe voltage generator circuit includes a first resistor, the specifiedvoltage being applied to one end of the first resistor and another endof the first resistor being coupled to an inverting input terminal ofthe differential amplifier, and a second resistor, one end of the secondresistor being coupled to the inverting input terminal of thedifferential amplifier and another end of the second resistor beingcoupled to an output terminal of the differential amplifier, wherein theAC signal is input to a non-inverting input terminal of the differentialamplifier, wherein the first AC voltage is output from the outputterminal of the differential amplifier, wherein one or both of the firstresistor and the second resistor is a variable resistor, and the drivecircuit further comprises a control circuit that controls a resistanceof one or both of the first resistor and the second resistor; and aphase adjuster that is placed between the signal source and thevoltage-to-current converter circuit or between the signal source andthe voltage generator circuit, and adjusts a phase of the AC signal,wherein the control circuit controls a phase adjustment amount of the ACsignal in the phase adjuster.
 2. A drive circuit comprising: a signalsource that outputs an AC signal; a voltage generator circuit thatincludes a differential amplifier that generates a first AC voltage witha constant amplitude from the AC signal and outputs the first AC voltageto one end of an external load; and a voltage-to-current convertercircuit that supplies an AC current with a constant amplitude inopposite phase to the first AC voltage to another end of the externalload in accordance with the AC signal, wherein the AC signal is apositive voltage varying with a constant amplitude with reference to aspecified voltage, wherein the voltage generator circuit includes afirst resistor, the AC signal being applied to one end of the firstresistor and another end of the first resistor being coupled to aninverting input terminal of the differential amplifier, and a secondresistor, one end of the second resistor being coupled to the invertinginput terminal of the differential amplifier and another end of thesecond resistor being coupled to an output terminal of the differentialamplifier, wherein the specified voltage is input to a non-invertinginput terminal of the differential amplifier, and wherein the first ACvoltage is output from the output terminal of the differentialamplifier.
 3. The drive circuit according to claim 2, wherein one orboth of the first resistor and the second resistor is a variableresistor, and the drive circuit further comprises a control circuit thatcontrols a resistance of one or both of the first resistor and thesecond resistor.
 4. The drive circuit according to claim 3, furthercomprising: a phase adjuster that is placed between the signal sourceand the voltage-to-current converter circuit or between the signalsource and the voltage generator circuit, and adjusts a phase of the ACsignal, wherein the control circuit controls a phase adjustment amountof the AC signal in the phase adjuster.
 5. A drive circuit comprising: asignal source that outputs an AC signal; a voltage generator circuitthat includes a differential amplifier that generates a first AC voltagewith a constant amplitude from the AC signal and outputs the first ACvoltage to one end of an external load; and a voltage-to-currentconverter circuit that supplies an AC current with a constant amplitudein opposite phase to the first AC voltage to another end of the externalload in accordance with the AC signal, wherein the AC signal is apositive voltage varying with a constant amplitude with reference to aspecified voltage. wherein the specified voltage is input to anon-inverting input terminal of the differential amplifier, the ACsignal is input to an inverting input terminal of the differentialamplifier, and the first AC voltage is output from an output terminal ofthe differential amplifier.
 6. A drive circuit comprising: a signalsource that outputs an AC signal; a voltage generator circuit thatincludes a differential amplifier that generates a first AC voltage witha constant amplitude from the AC signal and outputs the first AC voltageto one end of an external load; a voltage-to-current converter circuitthat supplies an AC current with a constant amplitude in opposite phaseto the first AC voltage to another end of the external load inaccordance with the AC signal; a phase adjuster that is placed betweenthe signal source and the voltage-to-current converter circuit orbetween the signal source and the voltage generator circuit, and adjustsa phase of the AC signal; and a control circuit that controls a phaseadjustment amount of the AC signal in the phase adjuster.
 7. A drivecircuit comprising: a signal source that outputs an AC signal; a voltagegenerator circuit that includes a differential amplifier that generatesa first AC voltage with a constant amplitude from the AC signal andoutputs the first AC voltage to one end of an external load; and avoltage-to-current converter circuit that supplies an AC current with aconstant amplitude in opposite phase to the first AC voltage to anotherend of the external load in accordance with the AC signal, wherein thesignal source outputs a first AC signal as the AC signal to thevoltage-to-current converter circuit, outputs a second AC signal as theAC signal to the voltage generator circuit, and is configured to be ableto adjust a phase of one or both of the first AC signal and the secondAC signal.